The present invention relates to an optical disc suitable for use in preservation, recording and reproduction of audio signals, images, information and the like, and also to a method of its manufacture.
The optical discs are generally composed of a pair of recording metal laminae each having grooves or pits shaped irregularities for servo tracking or the like, formed on a glass, transparent plastic or the like transparent replica substrate, with the same metal laminae disposed at a certain distance facing each other and bonded together with adhesive. Recently, increasing demands for such optical discs to increase the amount of information to be stored, to speed up the processing speed or the like are becoming greater at a rapid pace. In order to satisfy such demands, it is necessary to increase memory capacity and to rotate the optical discs at a higher speed.
Thereby, in lamination bonding of the substrates as processed as above in prior art optical discs, the following adhesives are used because of the simplicity, reduced-cost and the like in their manufacture. (1) thermoplastic hot-melt adhesives, (2) one-part anaerobic adhesives as disclosed in Japanese Patent Publication Laid-Open No. 61-151853, which was a reactive adhesive utilized to suppress disc deformation, and (3) two-part unmixture adhesives as disclosed in Japanese Patent Publication No. 61-50231.
These methods, however, involve the following problems, respectively. That is, (1) the thermoplastic hot-melt adhesive is inevitably accompanied with at least one of the defects resulting from heat when applying the adhesive or from pressure when lamination bonding, plastic or elastic deformation during a high speed operation, and warp or peeling due to aging during storage. Further with respect to the reactive adhesive, (2) the one-part anaerobic adhesive, as it cures in a second order and is bonded and laminated under normal pressures, allows air bubbles to form readily therein which are difficult to evacuate once included, thereby the same is cured with the air bubbles as included into a hard body which is too hard to absorb the air bubbles as much as sufficiently to relax them, thereby resulting in a poor flatness of the surface of the disc, or causing defects due to corrosion in the recording layer or medium due to uncured parts in the air bubbles. (3) The two-part unmixture adhesive is applied separately of the two-parts to a thickness of several tens .mu.m, respectively, then bonded together. At that time, as thus applied films are provided in a thick liquid form, there occurs a flow of the adhesive in the interface of lamination bonding due to the pressure applied at the time of lamination bonding, thereby causing uneven mixing thereof, thus, uneven curing, and further because of the hardness of the cured body, there occur wrinkles in the recording layer or medium, which becomes a cause of a poor performance in recording and reproduction.
As explained hereinabove, the prior art method (1) has the problem that there occur defects at least at the time of manufacture, at the time of utilization or at the time of storage of the optical discs. Further, with respect to the prior art methods (2) and (3), there arises a problem at the time of manufacture of the optical discs.